Ball transfer units (BTUs) are used for movement of pallets or cargo containers from one location to another. Generally, each ball transfer unit has a ball or roller, which projects above a "ball deck" and which is capable of freely rolling. The ball contacts a portion of the bottom surface of the container being conveyed along the deck. The container can be moved manually or by mechanical propulsion. The pallet or container is moveable along the ball deck by the reduced friction provided by the BTUs in the ball deck.
Prior art BTU assemblies typically include a cylindrical housing with an open top and a closed bottom having an aperture therein, a hemispherical cup seated in the housing, a plurality of ball bearings contained in the cup, and a ball seated on the ball bearings. The ball and ball bearings are retained in the cup and housing by a seal and retaining cover mounted to the housing. A typical prior art BTU is provided with a small drain hole in the center of the hemispherical cup to permit escape of moisture and liquids that penetrate the seal and that might contribute to corrosion of the ball bearings. Materials that escape from the cup through this drain hole then pass through the aperture in the bottom of the housing.
These typical prior art BTUs are extremely susceptible to contamination with dirt and other materials. In particular, if particulate or gummy contaminants or both become coated on the ball, the seal is often ineffective to remove the contaminants as the ball rotates, inevitably causing the contaminants to be collected inside the cup with the ball bearings, gumming up the ball bearings. This causes increased friction, decreasing the effectiveness of the prior art BTUs, and over time, these prior art BTUs will complete freeze up, i.e., it will not turn even when a force is applied. It has been found in typical applications such as airplane cargo, that prior art BTUs of this type must be replaced sometimes as often as once each year, and in very dirty applications, that replacements can be necessary as often as one time per month. This represents a significant cost in parts and labor.
What is desired, therefore, is a ball transfer unit in which the ball bearings will not become contaminated, thereby providing a substantially longer service life. Heretofore, improvements in a conventional BTU have been made in the seal mechanism, in an effort to prevent a contaminant from entering the unit. However, while these changes provided some improvements to service life, the improvements have not been significant.